This may be a silly question, but are you using Wanderer's Minuet? The Vault can be done with 100% WM uptime, with possibly the exception of breaking those tethered chains. As someone above suggested, the best way to handle the dps check on the final boss, is to DoT a few of them for bloodletter procs and begin to single target the rest.
At that level your priorities for single target should be Straight Arrow > DoTs/Iron Jaws > Empyreal Arrow > Bloodletter > Heavy Shot.
